Abbad bin Tamim (may Allah have mercy on him) narrates from his paternal uncle that he said: I saw the Messenger of Allah (peace and blessings be upon him) in the mosque lying straight on his back with one leg placed over the other.
Al-Zuhri (may Allah have mercy on him) says: Sa'id bin al-Musayyib (may Allah have mercy on him) informed me of the same from Umar and Uthman (may Allah be pleased with them both), and he could not count how often he had seen it from the two of them. Al-Zuhri (may Allah have mercy on him) says: People have come up with a grave matter.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3211